1. Field of the Invention
The present invention relates to an integrated phase-locked semiconductor laser, or simply integrated laser, having a plurality of waveguide paths arranged side by side therein, and a method of fabricating the same.
2. Description of the Prior Art
A device implemented by a laser of the type described is disclosed by Ikeda et al. in a paper entitled "Fundamental Transverse Mode and Light Output of Integrated Phase-Locked Laser", Technical Studies Report OQE86-64, The Institute of Electronics and Communication Engineers of Japan (1986).
An integrated laser has been elaborated to meet the demand for a semiconductor laser whose output is higher than the maximum output of a semiconductor laser of the type having a single waveguide path. However, a problem with an integrated laser is that integrating a plurality of waveguide paths makes it far more difficult to produce a single-peak far-field image, compared to a single waveguide path. To eliminate this problem, it is necessary that the waveguide paths individually oscillate in a fundamental mode and in a fundamental supermode of the same phase. One approach heretofore proposed for satisfying this condition is a structure which increases the difference in gain between the fundamental supermode and higher harmonic supermodes. A basic structure of this kind is discussed in the previously mentioned paper.
Referring to FIG. 8 of the drawings, a specific configuration of the prior art integrated laser disclosed in the above-mentioned paper is shown in a sectional view. As shown, the prior art integrated laser has a p-type GaAs substrate 31, an n-type GaAs current blocking layer 32, a p-type Al.sub.0.35 Ga.sub.0.65 As cladding layer 33, an Al.sub.0.08 Ga.sub.0.92 As active layer 34, an n-type Al.sub.0.35 Ga.sub.0.65 As cladding layer 35, and an n-type GaAs cap layer 36. The integrated laser shown in the figure is fabricated by sequentially depositing on the p-GaAs substrate 31 and n-GaAs current blocking layer 32 the p-Al.sub.0.35 Ga.sub.0.65 As cladding layer 33, Al.sub.0.08 Ga.sub.0.92 As active layer 34, n-Al.sub.0.35 Ga.sub.0.65 As cladding layer 35 and n-GaAs cap layer 36 by liquid-phase epitaxial growth. In the resulting laser, the individual waveguide paths have effective (or equivalent) refractive indexes n.sub.eff which are distributed as shown in FIG. 9.
In FIG. 9, the portions where the effective refractive index n.sub.eff is high, i.e., the waveguide path portions have a width W, while the other portions where it is low have a width S. Different values are selected for the widths W and S to change the difference in effective refractive index dn to thereby render the gain of the fundamental supermode higher than the gains of the higher harmonic supermodes. This allows the laser to oscillate in the fundamental supermode of the same phase.
FIGS. 10A and 10B are plots each showing calculated values of the gains of supermodes which are associated with the prior art integrated laser. Specifically, the graph shown in FIG. 10A was obtained with an effective refractive index difference dn of 1 percent, a stripe width W of 4 microns, a width S of 1 micron as defined by nearby stripes, and five stripes, while the graph of FIG. 10B was derived from an effective refractive index difference dn of 0.05 percent, a stripe with of 2 microns, a width S of 2 microns, and ten stripes.
Decreasing the widths W and S and the effective refractive index difference dn is successful in increasing the difference in gain of the highest harmonic supermode (.nu.=5 in the case of five stripes and .nu.=10 in the case of nine stripes) from the gain of the fundamental supermode (.nu.=1), as shown in FIG. 10B. This allows the transverse mode of the integrated laser to be controlled to the fundamental supermode, thereby implementing a single-peak far-field image. It is to be noted that, in FIG. 10B , the plat where .nu.=10 is cut off and therefore may not be taken into account.
However, when an integrated laser is to be fabricated by liquid-phase epitaxial growth, it is difficult to reduce the widths W and S. A prerequisite with the fabrication of an integrated laser by liquid-phase epitaxial growth is that the widths W and S be increased for the purpose of insuring a high output and reducing the light output per unit area of an emitting end. For this reason, the widths W and S have heretofore been limited to 3 to 4 microns and 1 to 2 microns, respectively, by the etching adapted to form the channels and the meltback which occurs during the course of liquid-phase epitaxial growth.